Two-pump optical parametric devices having reduced stimulated raman scattering noise levels

ABSTRACT

Two-pump optical parametric devices (OPDs), and methods of operating the same, generate desired output signals and idlers having reduced stimulated Raman scattering (SRS) noise levels. When the two-pump OPD is used as a two-pump optical parametric amplifier (OPA), the pumps are polarized perpendicular to each other, and the lower-frequency sideband (signal or idler) is polarized parallel to the lower-frequency pump (perpendicular to the higher-frequency pump). The desired output may be an amplified signal or a generated idler. When the two-pump OPD is used as a two-pump optical frequency converter (OFC), the pumps can be polarized parallel to one another, in which case the signal and idler are both perpendicular to the pumps, or perpendicular to one another, in which case the lower-frequency sideband (signal or idler) is polarized parallel to the lower-frequency pump (perpendicular to the higher-frequency pump).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to a continuation-in-part (CIP) U.S. patent application Ser. No. 11/068,555, filed Feb. 28, 2005, the teachings of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to optical parametric devices (OPDs), such as optical parametric amplifiers (OPAs) and optical frequency converters (OFCs), and more particularly to two-pump OPDs having reduced stimulated Raman scattering (SRS) noise levels in their output signals or idlers.

BACKGROUND OF THE INVENTION

Optical communication systems employ optical amplifiers, e.g., to compensate for signal attenuation in optical fibers. One type of amplifier that may be used in a fiber-based communication system is an OPA. As known in the art, an OPA is a device that produces a tunable coherent optical output via a nonlinear optical processes called four-wave mixing (FWM), in which two photons from one pump wave, or two pump waves, are destroyed and two new photons are created, with conservation of the total photon energy and momentum. The waves corresponding to the two new photons are usually referred to as the signal wave and the idler wave. This process amplifies a weak input signal and generates an idler, which is a frequency converted (FC) and phase-conjugated (PC) image of the signal. As known in the art, there is another type of FWM process, in which one signal photon and one pump photon are destroyed, and one idler photon and one different pump photon are produced, with conservation of the total photon energy and momentum. This process transfers power from the signal to the idler, which is a FC image of the signal. Optical frequency converters, OFCs, can be used to perform switching and routing in communication systems. The fundamentals of FWM are discussed in a book by G. P. Agrawal [“Nonlinear Fiber Optics, 3rd Edition,” Academic Press, 2001, hereafter referred to as GPA].

However, a problem with two-pump OPDs is the SRS noise that appears in the output signals and idlers. Because of SRS noise, the output signals or idlers produced by OPDs have lower signal-to-noise ratios (SNRs) than the input signals, which reduces the effectiveness of OPDs in communication systems.

SUMMARY OF THE INVENTION

In accordance with the present invention, two-pump OPDs, and methods of operating the same are described, which generate desired output signals and idlers having reduced SRS noise levels. In the case of a two-pump OPA, the pumps are polarized perpendicular to each other, and the lower-frequency sideband (signal or idler) is polarized parallel to the lower-frequency pump (perpendicular to the higher-frequency pump). The desired output may be an amplified signal or a generated idler (frequency-shifted copy of the signal). In the case of a two-pump OFC, the pumps can be polarized parallel to one another, in which case the signal and idler are both perpendicular to the pumps, or perpendicular to one another, in which case the lower-frequency sideband (signal or idler) is polarized parallel to the lower-frequency pump (perpendicular to the higher-frequency pump). The desired output may be an amplified signal or a generated idler.

More particularly, I describe a method of operating a two-pump optical parametric device, OPD, as an amplifier, OPA, that generates a desired output signal having a reduced stimulated Raman scattering, SRS, noise level. The method comprising the steps of

(1) applying first and second polarized pumps to the OPA, the frequency of the first pump, P₁, being lower than the frequency of the second pump, P₂, and the polarization of P₁ being perpendicular to the polarization of P₂;

(2) applying a polarized input signal S as an inner sideband adjacent to P₁ or P₂;

(3) outputting the desired output signal from an inner sideband adjacent to P₁ or P₂;

(4) wherein the inner sideband adjacent to P₁ is polarized parallel to P₁ and wherein

-   -   (a) when the desired output is an amplified signal S, the input         signal S is applied as an inner sideband adjacent to P₁ and the         SRS noise level in the desired output signal is reduced by         establishing the polarization of S to be perpendicular to the         polarization P₂ and     -   (b) when the desired signal is a generated PC idler, 2−, the         input signal S is applied as an inner sideband adjacent to P₂         and the SRS noise level in idler 2− is reduced by establishing         the polarization of S to be parallel to the polarization of P₂.

According to one embodiment, I describe a two-pump optical parametric amplifier, OPA, to generate a desired output signal having a reduced stimulated Raman scattering, SRS, noise level. The OPA comprises

a first polarized coupler for coupling a first pump, P₁, to the two-pump OPA;

a second polarized coupler for coupling a second pump, P₂, to the two-pump OPA, wherein the frequency of P₁ is lower than the frequency of P₂ and the polarization of the first polarized coupler is perpendicular to the polarization of the second polarized coupler;

a third polarized coupler for coupling an input signal S in an inner sideband adjacent P₁ or P₂; means for outputting the desired output signal (or idler) from an inner sideband adjacent to P₁ or P₂, and

wherein the inner sideband adjacent to P₁ is polarized parallel to P₁ and wherein

-   -   (a) when the desired output is an amplified signal S, the input         signal S is applied as an inner sideband adjacent to P₁ and the         SRS noise level in the desired output signal is reduced by         establishing the polarization of S to be perpendicular to the         polarization P₂ and     -   (b) when the desired signal is a generated PC idler, 2−, the         input signal S is applied as an inner sideband adjacent to P₂         and the SRS noise level in the idler 2− is reduced by         establishing the polarization of S to be parallel to the         polarization of P₂.

According to another aspect of the invention, I describe a method of operating a two-pump optical parametric device, OPD, as an optical frequency converter, OFC, to convert an input signal at a first frequency to a desired output idler I at a second frequency having a reduced stimulated Raman scattering, SRS, noise level. The method comprising the steps of:

(1) applying a first polarized pump, P₁, and a second polarized pump, P₂, to the OFC, the frequency of P₁ being lower than the frequency of P₂;

(2) applying a polarized input signal S as an inner sideband 1+ adjacent to P₁ or an outer sideband 2+ adjacent to P₂; and

(3) outputting the desired output idler I from an outer sideband 2+ adjacent to P₂ or an inner sideband 1+ adjacent to P₁.

In another embodiment, I describe a two-pump optical frequency converter, OFC, for converting an input signal at a first frequency to a desired output idler at a second frequency having a reduced stimulated Raman scattering, SRS, noise level. The OFC comprises

a first polarized coupler for coupling a first pump, P₁, to the two-pump OFC;

a second polarized coupler for coupling a second pump, P₂, to the two-pump OFC, wherein the frequency of P₁ is lower than the frequency of P₂;

a third polarized coupler for coupling an input signal S in an inner sideband 1+ adjacent to P₁ or an outer sideband 2+ adjacent to P₂; and

means for outputting the desired output idler, I, from an outer sideband 2+ adjacent to P₂ or an inner sideband 1+ adjacent to P₁.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which:

FIG. 1 shows a two-pump OPD in accordance with one embodiment of the present invention;

FIG. 2 illustrates a representative frequency structure in the OPD of FIG. 1;

FIG. 3 shows the spectral (frequency) characteristics of an input signal (S) that is simultaneously amplified and replicated within four output spectral bands. If the signal is in band 1+, the 1−, 2− and 2+ bands are referred to as idler (I) bands;

FIG. 4 shows an illustration of the SRS noise signals (SRS1 and SRS2) for a two pump OPA, where the two pumps P1 and P2, the input signal S in band 1+, and idler I in band 2− are all parallel polarized;

FIG. 5 shows the input signal S in band 2− and the idler I in band 1+, reversed from the positions shown in FIG. 4;

FIG. 6 shows the SRS2 noise when the polarization of the input signal S in band 1+ is parallel to pump P2;

FIG. 7 shows the SRS2 noise when the polarization of the input signal S in band 1+ is perpendicular to pump P2;

FIG. 8 shows the SRS2 noise when the polarization of the input signal S in band 2− is parallel to pump P2;

FIG. 9 shows the SRS2 noise when the polarization of the input signal S in band 2− is perpendicular to pump P2;

FIG. 10 shows the general frequency characteristics of the invented OPA, in which the desired output signal is in band 1+ (or 2−) and an idler is in band 2− (1+);

FIGS. 11A through 11C show the decrease in n₁₊ as the idler 1+ increases in frequency from the frequency of pump P1 to the frequency of pump P2;

FIG. 12 shows that when the pumps P1 and P2 are parallel and idler 1+ is perpendicular to P1, then n₁₊ is zero and idler 2+ is perpendicular to P1;

FIG. 13 shows that when the pump P1 is perpendicular to pump P2 and idler 1+ is parallel to P1, then n₁₊ and R₂₊ are zero and idler 2+ is perpendicular to P1; and

FIG. 14 shows that when the pump P1 is perpendicular to pump P2 and idler 1+ is perpendicular to P1, then no idler 2+ is generated.

DETAILED DESCRIPTION

FIG. 1 shows, in accordance with one embodiment of the present invention, a two-pump OPD 100 that is configured for use in a long-haul transmission line of an optical communication system. OPA 100 is coupled between two sections 102 and 102′ of long-haul optical fiber. The input optical signal S from section 102 is received or coupled via polarization coupler 103 to a coupler 104 of the OPA 100. The coupler 104 is configured to combine an input signal S from section 102 with two different pump waves generated by two pump-wave sources (e.g., lasers) 111 and 112. The output of pump sources 111 and 112 are received or coupled via polarization couplers 113 and 114, respectively, and are combined in coupler 115. The combined two-pump signal P₁+P₂ from coupler 115 is then further combined with input signal S in coupler 104. Depending on the implementation of OPD 100, each of the pump wave sources 111 and 112 may be a continuous-wave (CW) or pulsed optical field. The combined optical output from coupler 104 is coupled into a highly nonlinear fiber (HNF) 108, in which the signal is amplified by FWM. Frequency shifted copies of the signal are also produced. While the present invention describes the use of HNF 108 for OPA and OFC, it should be understood that other χ⁽³⁾ media that exhibit FWM, such as Kerr media, could be used. Additionally, a three-way coupler may be used to replace couplers 104 and 115 to combine the input signal S with the two pumps P₁ and P₂. A filter 109 is placed at the end of HNF 108 to pass the desired output optical sideband 110 (amplified signal or generated idler) and block the undesired sidebands. For example, if the desired output sideband 110 is an amplified version of input signal S from fiber 102, then filter 109 passes that amplified signal S and filters out the pump signals P₁ and P₂, and the one or more idlers generated in the HNF 108. Conversely, if the desired output sideband 110 is one of the idlers, then filter 109 filters out the pump signals P₁ and P₂, the amplified signal S from fiber 102, and the undesired idlers generated in the HNF 108. The desired output signal 110 is then transmitted over section 102′. One attractive feature of a two-pump OPD 100 is that the desired output sideband 110 can be selected to be an amplified or non-amplified, and conjugated or non-conjugated, version of input signal S at an arbitrary frequency (wavelength). In accordance with the present invention, as will be discussed in more detail in later paragraphs, the polarization of each of the couplers 103, 113, and 114, is selected to control the polarization of the input signal S, pump 111, and pump 112, respectively, so as to minimize SRS noise in the desired output signal 110 of OPD 100.

FIG. 2 shows a simplified diagram of the output frequency structure developed in HNLF 108 of OPD 100. In addition to two pumps labeled P₁ and P₂ and located at frequencies ω₁ and ω₂, respectively, and a signal S (illustratively a sideband at frequency ω₁₊), various FWM processes in HNLF 408 produce three complementary sidebands at frequencies ω¹⁻, ω²⁻, and ω₂₊. In general, the frequency of the signal S may be at any one of the four sidebands, with the remaining three sidebands being generated by FWM processes in OPD 100.

The following paragraphs describe the FWM processes in OPD 100 leading to the frequency structure of FIG. 2. Suppose that the optical signal S is at frequency c and the remaining three sidebands ω¹⁻, ω²⁻, and ω₂₊ are idler sidebands. Then a modulation interaction (MI) produces the first idler sideband at frequency c, according to the frequency-matching condition

2ω₁=ω¹⁻+ω₁₊,  (1)

a phase-conjugated (PC) process produces a second idler sideband at frequency w 2− according to the frequency-matching condition

ω₁+ω₂=ω₁₊+ω²⁻,  (2)

and a Bragg scattering (BS), or frequency converter (FC), process produces a third idler sideband at frequency ω₂₊ according to the frequency-matching condition

ω₁₊+ω₂=ω₁+ω₂₊.  (3)

In addition, each of the three idler sidebands is coupled to the other two idler sidebands by an appropriate FWM process, i.e., MI, BS, or PC, which obeys an equation analogous to Eq. (1), (2) or (3).

In addition to the sidebands illustrated in FIG. 2, OPD 100 may also generate several additional sidebands (not shown). For example, MI with P₂ generates additional sidebands with frequencies 2ω₂−ω¹⁻ and 2ω₂−ω₁₊ and MI with P1 generates additional sidebands with frequencies 2ω₁−ω²⁻ and 2ω₁−ω₂₊. However, unlike the four original sidebands shown in FIG. 2, each of which is coupled to all of the other three, none of the additional sidebands is coupled to all of the original four or all of the other three additional sidebands. Furthermore, for most values of ω₁₊, the additional sidebands are driven non-resonantly. Consequently, the effects of the additional sidebands on the operation of OPD 100 are not considered further.

Since OPD 100 relies on FWM enabled by the Kerr effect to amplify and generate sidebands, the first and second pumps (P₁ and P₂) are not required to be applied to HNF 108 in a prescribed order; ω₁ and ω₂ are not required to be derived based on or have a specific relationship with the specific energy-level transitions of the material of the HNF 108; and neither pump power is required to exceed the level that produces electromagnetically induced transparency (EIT) in HNF 108.

With reference to FIG. 3, there are shown illustrative frequency characteristics of a modulated input signal S, in band 1+, that is simultaneously amplified and replicated within four spectral bands 1−, 1+, 2− and 2+. The generated idlers are either spectrally-mirrored images of the modulated input signal S (idler bands 1− and 2−) or a translated (frequency-shifted) replica (idler band 2+). The spectrally-mirrored idlers are PCs, which offer the potential for mitigating impairments. The four signal bands produced by two-pump OPD 100 allow for considerable flexibility in selecting the properties of the desired output signal or idler. An inner band placement of input signal S (i.e., in bands 1+ and 2− located between the pumps P₁ and P₂) generates both an outer band nonPC (replica) and PCs located in the inner and outer bands. Thus, as shown in FIG. 3, an input signal S in band 1+ produces a nonPC in band 2+ and PCs in both inner band 2− and outer band 1−. An outer band placement of input signal S (i.e., in bands 1− and 2+) generates both an inner band nonPC (replica) and PCs located in the inner and outer bands. The existence of multiple bands depends on the presence of both pumps P₁ and P₂. The frequency of pumps P₁ and P₂ can be tuned in ways such that the signals and idlers in all four bands are strongly coupled, the signal and idler in bands 1+ and 2− are strongly coupled (OPA enabled by PC), or the signal and idler in bands 1+ and 2+ are strongly coupled (OFC enabled by BS).

Consider OPA enabled by phase-conjugated (PC) process. In this process γ₁+γ₂→γ₁₊+γ²⁻: Two pump photons (γ) are destroyed (one from each pump), and one signal and one idler photon are created. OPA is characterized by the input-output relations

A ₁₊(z)=μ(z)A ₁₊(0)+ν(z)A ²⁻(0)*,  (4)

A ²⁻(z)*=ν(z)*A ₁₊(0)+μ(z)*A ²⁻(0)*,  (5)

where A₁₊ and A²⁻ are the amplitudes of the 1+ and 2− sidebands, respectively, and the transfer functions satisfy the auxiliary equation |μ|²−|ν|²=1 [C. J. McKinstrie, S. Radic and M. G. Raymer, “Quantum noise properties of parametric amplifiers driven by two pump waves,” Opt. Express 12, 5037-5066 (2004), hereafter referred to as MRR]. One can model the effects of SRS noise (approximately) by adding random (and independent) amplitude fluctuations δa to each of the input amplitudes. Because ω₁₊ and ω²⁻ are both less than ω₂, δa₁₊ and δa²⁻ are both nonzero (unless 1+ or 2− is perpendicular to P₂). If the input consists solely of noise, the outputs

R ₁₊(z)=|A ₁₊(z)|²=|μ(z)|² |δa ₁₊|²+|ν(z)|² |δa ²⁻|²=|μ(z)|² n ₁₊+|ν(z)|² n ²⁻,  (6)

R ²⁻(z)=|A ²⁻(z)|²=|ν(z)|² |δa ₁₊|²+|μ(z)|² |δa ²⁻|²=|ν(z)|² n ₁₊+|μ(z)|² n ²⁻,  (7)

where n₁₊ and n₂ are the input noise powers, and R₁₊ and R²⁻ are the output noise powers, respectively. The SRS noise photons at ω₊ are amplified by FWM, which also couples the noise photons at ω²⁻ to the output at ω₁₊. A similar statement can be made about the output at ω²⁻. These equations imply that R₁₊−R²⁻=n₁₊−n²⁻>0 (unless 1+ is perpendicular to P₂). The gain G=|μ|². The auxiliary equation implies that |ν|²=G−1. In the high-gain regime (G>>1), |ν|²≈|μ|², and R₁₊≈R²⁻≈G(n₁₊+n²⁻). Thus, as a general rule, one can minimize the noise in both outputs by setting 1+ perpendicular to P₂, in which case n₁₊=0 [R. H. Stolen, “Polarization effects in fiber Raman and Brillouin lasers,” IEEE J. Quantum. Electron. 15, 1157-1160 (1979), hereafter referred to as RHS].

With reference to FIG. 4, there is shown an illustration of the resulting SRS noise fields SRS1 and SRS2 for a two-pump OPA 100, where the two pumps P₁ and P₂, input signal S (sideband 1+) and idler II (sideband 2−) are all parallel polarized. As shown, the SRS1 noise field lies in the same plane as the pump P₁ and the SRS2 noise field lies in the same plane as the pump P₂. Since pumps P₁ and P₂ are parallel (i.e., both are shown vertically polarized) SRS1 and SRS2 are in the same plane. Note that the amplitudes of the SRS1 and SRS2 noise fields increase to a well-defined peak values with increasing frequency separation from pumps P₁ and P₂, respectively. (For example, if the wavelength of pump P₂ is 1440 nm, then SRS2 peaks at about 110 nm from the pump wavelength, at about 1550 nm.) Notice that in the example of FIG. 4, since the frequencies of the input signal S and idler I lie between the frequencies of pumps P₁ and P₂, the SRS1 noise field has no direct effect on these signals. Thus, since it is only the SRS2 noise field that affects directly the desired output signal or idler 110 of OPA 100, the effects of the SRS1 noise field will not be considered further. It follows from Eqs. (6) and (7) that R₁₊=Gn₁₊+(G−1)n²⁻ and R²⁻=(G−1)n₁₊+Gn²⁻. Because noise photons at both input frequencies are coupled to both outputs, the output noise powers of the sidebands are comparable. However, R2− is slightly lower than R1+, as stated above. In this configuration, the idler is the desired output.

With reference to FIG. 5, there is shown the input signal S (in band 2−) and idler I (in band 1+) reversed from the positions shown in FIG. 4. Once again, it follows from Eqs. (6) and (7) that R₁₊=Gn₁₊+(G−1)n₂ and R²⁻=(G−1)n₁₊+Gn²⁻. Because noise photons at both input frequencies are coupled to both outputs, the output noise powers of the sidebands are comparable. However, R2− is slightly lower than R1+, as stated above. In this configuration, the signal is the desired output.

In accordance with the present invention, I have recognized that the SRS growth rate g_(R), and the amplified noise field that results, is polarization dependent. As stated in [RHS], the SRS growth rate of a signal that is perpendicular to the pump is an order-of-magnitude lower that the growth rate of a signal that is parallel to the pump. Since the output amplitude A(z)=A(0)exp(g_(R)z), an order-of-magnitude difference in the gain exponent g_(R)z causes a many-orders-of magnitude difference in the gain exp(g_(R)z) and, hence, in the output amplitude A(z): For practical purposes, the SRS noise field that is perpendicular to pump 2 can be neglected. Hence, in FIGS. 6-9 the noise field SRS2 is drawn parallel to pump 2.

In FIGS. 6-9 and the discussion that follows, two signals are said to be parallel if both signals are vertical or both signals are horizontal. Similarly, two signals are said to be perpendicular (orthogonal) if one signal is vertical and the other is horizontal, or vice-versa. The concept of orthogonality is not limited to the linearly-polarized states illustrated in the figures. For example, right-circularly-polarized and left-circularly-polarized states are also orthogonal, even though neither state is linearly polarized [C. J. McKinstrie, H. Kogelnik, R. M. Jopson, S. Radic and A. V. Kanaev, “Four-wave mixing in fibers with random birefringence,” Opt. Express 12, 2033-2055 (2004), hereafter referred to as MKJRK]. Although these figures were drawn for linearly-polarized states (horizontal and vertical), they also represent more-general polarization states that are parallel or orthogonal. FIG. 6 shows the input signal S in band 1+ polarized in direction X and the idler I in band 2− polarized in direction Y. FIG. 7 shows the input signal S in band 1+ polarized in direction Y and the idler I in band 2− polarized in direction X. FIG. 8 shows the input signal S in band 2− polarized in direction X and the idler I in band 1+ polarized in direction Y. FIG. 9 shows the input signal S in band 2− polarized in direction Y and the idler I in band 1+ polarized in direction X. In FIGS. 6-9 the polarization of the generated idler I is perpendicular to the polarization of the input signal S [see reference MKJRK], regardless of whether the idler frequency is higher or lower than the signal frequency. In addition, the pumps P1 and P2 are perpendicular. (If pumps P1 and P2 were parallel, a perpendicular input signal S would not generate an idler [MKJRK]. This configuration is not useful.) The parametric gain produced by FWM is polarization-independent [MKJRK]: It is the same regardless of whether the input signal S is parallel or perpendicular to pump P₁ (or P₂). For the configuration shown in FIG. 6, R₁₊=Gn₁₊ and R²⁻=(G−1)n₁₊. The output idler (2−) has slightly less noise than the output signal (1+), but both are noisy. For the configuration shown in FIG. 7, R₁₊=(G−1)n²⁻ and R₂=Gn²⁻. The output signal (1+) has slightly less noise than the output idler (2−), but neither is noisy. If the desired output is the 1+ signal, the second configuration is better (because G−1<G and n²⁻<<n₁₊). If the desired output is the 2− idler, the second configuration is better (because G−1≈G and n²⁻<<n₁₊).

For the configuration shown in FIG. 8, R₁₊=(G−1)n₂ and R₂=Gn²⁻. The output idler (1+) has slightly less noise than the output signal (2−), but neither is noisy. For the configuration shown in FIG. 9, R₁₊=Gn₁₊ and R₂=(G−1)n₁₊. The output signal (2−) has slightly less noise than the output idler (1+), but both are noisy. If the desired output is the 2− signal, the first configuration is better (because G−1≈G and n₂<<n₁₊). If the desired output is the 1+ idler, the first configuration is better (because G−1<G and n₂<<n₁₊).

It follows from the analyses of FIGS. 6-9 that, if OPA 100 is to be operated as a low-noise device, the higher-frequency sideband should be parallel to pump 2 (so the noise source is n2−). In this case the lower-frequency sideband has slightly less noise, but neither sideband is noisy. This optimal configuration is illustrated in FIG. 10.

Now consider OFC enabled by BS. In this process γ₁₊+γ₂→γ₁+γ₂₊: One pump and one signal photon are destroyed and one pump and one idler photon are created. OFC is characterized by the input-output relations

A ₁₊(z)=μ(z)A ₁₊(0)+ν(z)A ₂₊(0),  (8)

A ₂₊(z)=−ν*(z)A ₁₊(0)+μ(z)*A ₂₊(0),  (9)

where the transfer functions satisfy the auxiliary equation |μ|²+|ν|²=1 [MRR]. As before, consider the effects of SRS noise, which are modeled (approximately) as random amplitude fluctuations δa added to the input amplitudes. Because ω₂₊>ω₂, δa₂₊=0. It follows from this fact that, if the input consists solely of noise,

R ₁₊(z)=|A ₁₊(z)|²=|μ(z)|² |δa ₁₊|²=|μ(z)|² n ₁₊,  (10)

R ₂₊(z)=|A ₂₊(z)|²=|ν(z)|² |δa ₁₊|²=|ν(z)|² n ₁₊,  (11)

where R₂₊ is the output SRS noise power at frequency ω₂₊.

First, suppose that 1+ is the signal and 2+ is the idler. Then the output noise R₂₊=|ν|²n₁₊. Because the 2+ idler is desired (A₂₊=−ν*A₁₊), in a typical experiment |ν|²≈1 and, hence, |μ|²≈0. If the pumps are parallel and 1+ is parallel to P₁, then R₂₊ is always nonzero. As ω₁₊ increases from ω₁ to ω₂, n₁₊ decreases, as illustrated in FIG. 11. If the pumps are parallel and 1+ is perpendicular to P₁, as illustrated in FIG. 12, then n₁₊=0, 2+ is generated perpendicular to P₁ [see MKJRK] and R₂₊=0: SRS noise is eliminated completely. If P₁ and P₂ are perpendicular and 1+ is parallel to P₁ (perpendicular to P₂), as illustrated in FIG. 13, then n₁₊=0, 2+ is generated perpendicular to P₁ (parallel to P₂) [MKJRK] and R₂₊=0: SRS noise is eliminated completely. If P₁ and P₂ are perpendicular and 1+ is perpendicular to P₁ (parallel to P₂), as illustrated in FIG. 14, no 2+ idler is generated [MKJRK].

Second, suppose that 2+ is the signal and 1+ is the idler. Then the output noise R+=|μ|²≈n₁₊. Because the 1+ idler is desired (A₁₊=νA₂₊), in a typical experiment |ν|²≈1 and, hence, |μ|²≈0: Most SRS noise photons at ω₁₊ are frequency shifted to ω₂₊. Few remain at ω₁₊ to pollute the idler. SRS noise is eliminated completely if the pumps are parallel and 2+ is perpendicular to P₂, in which case 1+ is generated perpendicular to P₂ [MKJRK], or if the pumps are perpendicular and 2+ is parallel to P₂ (perpendicular to P₁), in which case 1+ is parallel to P₁ (perpendicular to P₂) [MKJRK]. If the pumps are perpendicular and 2+ is perpendicular to P₂ (parallel to P₁), no 1+ idler is generated [MKJRK].

It follows from the analyses of FIGS. 11-14 that, if OFC 100 is to be operated as a low-noise device, the lower-frequency sideband should be perpendicular to pump 2 (so the noise source is zero). In this case neither sideband has noise. These optimal configurations were illustrated in FIGS. 12 and 13.

Various modifications of the described embodiments, as well as other embodiments of the inventions (OPAs and OFCs), which are apparent to persons skilled in the art to which the inventions pertain, are deemed to lie within the principle and scope of the inventions as expressed in the following claims.

Although the steps in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those steps, those steps are not necessarily intended to be limited to being implemented in that particular sequence. 

1. A method of operating a two-pump optical parametric device, OPD, as an amplifier, OPA, to generate a desired output signal having a reduced stimulated Raman scattering, SRS, noise level, comprising the steps of: applying first and second polarized pumps to the OPA, the frequency of the first pump, P₁, being lower than the frequency of the second pump, P₂, and the polarization of P₁ being perpendicular to the polarization of P₂; applying a polarized input signal S as an inner sideband adjacent to P₁ or P₂; outputting the desired output signal from an inner sideband adjacent to P₁ or P₂; wherein the inner sideband adjacent to P₁ is polarized parallel to P₁ and wherein (a) when the desired output is an amplified signal S, the input signal S is applied as an inner sideband adjacent to P₁ and the SRS noise level in the desired output signal is reduced by establishing the polarization of S to be perpendicular to the polarization P₂ and (b) when the desired signal is a generated PC idler, 2−, the input signal S is applied as an inner sideband adjacent to P₂ and the SRS noise level in idler 2− is reduced by establishing the polarization of S to be parallel to the polarization of P₂.
 2. The method of claim 1, wherein the wavelengths of P₁ and P₂ are selected to minimize the outer-band idlers 1− and 2+.
 3. A two-pump optical parametric amplifier, OPA, to generate a desired output signal having a reduced stimulated Raman scattering, SRS, noise level, comprising: a first polarized coupler for coupling a first pump, P₁, to the two-pump OPA; a second polarized coupler for coupling a second pump, P₂, to the two-pump OPA, wherein the frequency of P₁ is lower than the frequency of P₂ and the polarization of the first polarized coupler is perpendicular to the polarization of the second polarized coupler; a third polarized coupler for coupling an input signal S in an inner sideband adjacent P₁ or P₂; means for outputting the desired output signal (or idler) from an inner sideband adjacent to P₁ or P₂, and wherein the inner sideband adjacent to P₁ is polarized parallel to P₁ and wherein (a) when the desired output is an amplified signal S, the input signal S is applied as an inner sideband adjacent to P₁ and the SRS noise level in the desired output signal is reduced by establishing the polarization of S to be perpendicular to the polarization P₂ and (b) when the desired signal is a generated PC idler, 2−, the input signal S is applied as an inner sideband adjacent to P₂ and the SRS noise level in the idler 2− is reduced by establishing the polarization of S to be parallel to the polarization of P₂.
 4. The two-pump OPA of claim 3, wherein the frequencies of P₁ and P₂ are selected to minimize the outer-band idlers 1− and 2+.
 5. A method of operating a two-pump optical parametric device, OPD, as an optical frequency converter, OFC, to convert an input signal at a first frequency to a desired output idler I at a second frequency having a reduced stimulated Raman scattering, SRS, noise level, comprising the steps of: applying a first polarized pump, P₁, and a second polarized pump, P₂, to the OFC, the frequency of P₁ being lower than the frequency of P₂; applying a polarized input signal S as an inner sideband 1+ adjacent to P₁ or an outer sideband 2+ adjacent to P₂; and outputting the desired output idler I from an outer sideband 2+ adjacent to P₂ or an inner sideband 1+ adjacent to P₁.
 6. The method of claim 5, wherein the frequencies of P₁ and P₂ are selected to minimize the idlers 1− and 2−.
 7. The method of claim 5, wherein P₁ and P₂ are parallel, and the signal S and idler I are both perpendicular to P₁ and P₂.
 8. The method of claim 5, wherein P₁ and P₂ are perpendicular, and the inner sideband 1+ is parallel to P₁.
 9. A two-pump optical frequency converter, OFC, for converting an input signal at a first frequency to a desired output idler at a second frequency having a reduced stimulated Raman scattering, SRS, noise level, comprising: a first polarized coupler for coupling a first pump, P₁, to the two-pump OFC; a second polarized coupler for coupling a second pump, P₂, to the two-pump OFC, wherein the frequency of P₁ is lower than the frequency of P₂; a third polarized coupler for coupling an input signal S in an inner sideband 1+ adjacent to P₁ or an outer sideband 2+ adjacent to P₂; and means for outputting the desired output idler, I, from an outer sideband 2+ adjacent to P₂ or an inner sideband 1+ adjacent to P₁.
 10. The two-pump OFC of claim 5, wherein P₁ and P₂ are parallel, and the signal S and idler I are both perpendicular to P₁ and P₂.
 11. The two-pump OFC of claim 9, wherein P₁ and P₂ are perpendicular, and the inner sideband 1+ is parallel to P₁.
 12. The two-pump OFC of claim 9, wherein the frequencies of P₁ and P₂ are selected to minimize the idlers 1− and 2−. 